For those who would doubt that it is legal in black and white, I cite from the BATF FAQ of their "10 most common questions:"

"Firearms may be lawfully made by persons who do not hold a manufacturer’s license under the GCA provided they are not for sale or distribution and the maker is not prohibited from receiving or possessing firearms. However, a person is prohibited from assembling a non-sporting semiautomatic rifle or shotgun from 10 or more imported parts, as set forth in regulations in 27 C.F.R. 478.39. In addition, the making of an NFA firearm requires a tax payment and advance approval by ATF. An application to make a machinegun will not be approved unless documentation is submitted showing that the firearm is being made for the official use of a Federal, State, or local government agency (18 U.S.C. § 922(o),(r); 26 U.S.C. § 5822; 27 C.F.R. §§ 478.39, 479.62, and 479.105)."